/*
 * TeleStax, Open Source Cloud Communications
 * Copyright 2011-2014, Telestax Inc and individual contributors
 * by the @authors tag.
 *
 * This program is free software: you can redistribute it and/or modify
 * under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation; either version 3 of
 * the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 *
 */

package org.restcomm.media.core.rtp;

import java.util.UUID;

/**
 * Generates SSRC identifiers for an RTP Channel.
 * 
 * @author Henrique Rosa (henrique.rosa@telestax.com)
 * 
 */
public class SsrcGenerator {

	public static final int MAX_SIZE = 32;

	public static long generateSsrc() {
		UUID uuid = UUID.randomUUID();
		long leastSignificantBits = uuid.getLeastSignificantBits();
		byte[] data = uIntLongToByteWord(leastSignificantBits);
		return bytesToUIntLong(data, 0);
	}

	/**
	 * Combines four bytes (most significant bit first) into a 32 bit unsigned
	 * integer.
	 * 
	 * @param bytes
	 * @param index
	 *            of most significant byte
	 * @return long with the 32 bit unsigned integer
	 */
	static long bytesToUIntLong(byte[] bytes, int index) {
		long accum = 0;
		int i = 3;
		for (int shiftBy = 0; shiftBy < 32; shiftBy += 8) {
			accum |= ((long) (bytes[index + i] & 0xff)) << shiftBy;
			i--;
		}
		return accum;
	}

	/**
	 * Converts an unsigned 32 bit integer, stored in a long, into an array of
	 * bytes.
	 * 
	 * @param j
	 *            a long
	 * @return byte[4] representing the unsigned integer, most significant bit
	 *         first.
	 */
	static byte[] uIntLongToByteWord(long j) {
		int i = (int) j;
		byte[] byteWord = new byte[4];
		byteWord[0] = (byte) ((i >>> 24) & 0x000000FF);
		byteWord[1] = (byte) ((i >> 16) & 0x000000FF);
		byteWord[2] = (byte) ((i >> 8) & 0x000000FF);
		byteWord[3] = (byte) (i & 0x00FF);
		return byteWord;
	}

}
